Turbocharged compound cycle ducted fan engine system

ABSTRACT

A turbocharged, compounded cycle ducted fan engine system includes a conventional internal combustion engine drivingly connected to a fan enclosed in a duct. The fan provides propulsive thrust by accelerating air through the duct and out an exhaust nozzle. A turbocharger is disposed in the duct and receives a portion of the air compressed by the fan. The turbocharger compressor further pressurizes the air and directs it to the internal combustion engine where it is burned and exits as exhaust gas to drive the turbine. A power turbine also driven by exhaust gas is also drivingly connected through the engine to the fan to provide additional power. The size and weight of the turbocharger are reduced since the compressor&#39;s work is partially achieved by the compression effect of the fan. The total propulsive thrust includes the fan generated thrust which bypasses the turbocharger and the thrust of exhaust gases exiting the turbine.

This is a divisional of co-pending application Ser. No. 07/017,825 filedon Feb. 24, 1987, U.S. Pat. No. 4,815,282.

BACKGROUND OF THE INVENTION I. Field of the Invention

This invention relates to propulsion systems for aircraft and, inparticular, to a turbocharged internal combustion engine compounded witha gas turbine to drive a ducted fan to generate the required propulsivethrust for most efficient operation at high subsonic flight speedsthrough the upper limits of the tropopause into the lower levels of thestratosphere.

II. Description of the Prior Art

It has been previously recognized that internal combustion engines aremore fuel efficient than gas turbine engines in aircraft.

It has been previously recognized that internal combustion engines aremore fuel efficient than gas turbine engines in aircraft. The specificfuel consumption of a conventional gas turbine increases with altitudeand is also higher at part load as compared to the internal combustionengine. Well designed turbocharged piston engines are capable ofproviding a specific fuel consumption at high altitudes equal to orbetter than that available at sea level, with part load specific fuelconsumption also equal to or better than that attained at maximum load.

However, gas turbines have largely replaced internal combustion enginesfor low altitude operation due to their smaller size and lighter weightfor a given thrust level. Although this size and weight advantagediminishes with ascending altitude, gas turbines have been thepowerplant normally used for high altitude high speed flight due to theabsence of a more efficient, lighter weight option.

It has also been recognized that propellers or propulsors whichmoderately accelerate a large volume of air are highly efficient atflight speeds of about mach 0.6 or less, while turbo fans which greatlyaccelerate a smaller volume of air are less efficient at such low airspeeds, but increase in efficiency as flight speed increases. Therefore,for high subsonic or supersonic flight, turbo fans or turbo jets havepreviously been the preferred propulsion systems.

SUMMARY OF THE PRESENT INVENTION

This invention provides an improvement over previously known turbo fanpropulsion systems by compounding a highly fuel efficient internalcombustion engine with a gas powered turbine. The internal combustionengine may be of either the spark ignition or the compression ignitiontype. The outputs of the combustion engine and the power turbine areconnected through appropriate drive transmissions to drive a fan. Thefan provides the bulk of the propulsive thrust necessary for aircraftflight. In the preferred embodiment, the fan is enclosed in a duct.

The combustion engine is turbocharged in order to provide sea levelhorsepower to high altitudes. For this purpose, a turbocharger,consisting of an exhaust gas driven turbine connected to a compressor bya drive shaft is included. The compressor generates sufficient airpressure to turbocharge the combustion engine. The fan, the compressorand the turbine each may be single or multiple stage as desired, and thepower turbine preferably is a free turbine not mechanically linked tothe turbocharger shaft. A conventional heat exchanger coolant system isused for the combustion engine and waste gates may be provided so thatexhaust gas can bypass either the power turbine or the turbocharger, orboth, if desired.

In the present invention, the turbocharger is disposed downstream fromthe propulsive fan. This enables a compressor of smaller weight anddimension to produce the pressure required to turbocharge the combustionengine. Thus, the entire engine system is lighter, more streamlined andmore efficient. In the preferred embodiment, the fan and turbochargerare coaxial and the combustion engine is located closely adjacent theturbocharger to minimize power losses in drive trains and heat losses ingas conduits.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be more fully understood by reference to thefollowing detailed description when read in conjunction with theaccompanying drawing, in which like reference characters refer to likeparts throughout the several views, and in which:

FIG. 1 is a schematic diagram of the components of the presentinvention;

FIG. 2 is a schematic diagram of an alternative embodiment of thepresent invention;

FIG. 3 is a schematic diagram of another alternative embodiment of thepresent invention; and

FIG. 4 is a side diagrammatic view of a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, the engine system 10 is thereshown indiagrammatic form, generally comprising an internal combustion engine12, a fan 14, a power turbine 16 and a turbocharger 18 comprising acompressor 20 and a turbine 22 coaxially mounted on a common shaft 24.The combustion engine 12 may be of the spark ignition type or thecompression ignition type and includes an air inlet 26 and an exhaustoutlet 28. The combustion engine 12 further includes a drive shaft 30which is connected via an appropriate first drive transmission 32 to ashaft 34 of the fan 14. The drive shaft 30 is also connected through anappropriate second drive transmission 36 to a shaft 38 of the powerturbine 16.

The fan 14 may be of one or more stages and may include blades havingvariable angular adjustments for variable thrust generation andpreferably includes a duct 40 and an inlet diffuser 42. The duct 40 maybe elongated as shown in FIG. 4 to include the turbocharger 18 so thatsome of the air flow leaving the fan 14 can enter the compressor 20. Thecompressor 20 then further pressurizes the air and exhausts it throughconduit 44 and intercooler 46 to the engine inlet 26. In the preferredembodiment, as shown in FIG. 4, the compressor 20 and the ducted fan 14are coaxial, and are concentric with the elongated duct 40. They can besusported in the duct by struts such as at 47.

According to a process well known in the art, the air is furthercompressed by the piston stroke of the combustion engine 12, and mixedwith fuel. The resulting mixture is ignited and exhausted following thepower stroke through exhaust outlet 28 at high pressure and temperature.The exhaust gases leaving the combustion engine 12 are conducted viaconduit 48 to a nozzle 50 where they are directed to impinge on theblades of the power turbine 16. Thereafter, the gases may be led furtherby a conduit 52 and a nozzle 54 to impinge on the blades of the turbine22 Finally, the exhaust gases are directed out of the engine system 10through the exhaust nozzle 56. Preferably, the gas conduits 48 and 52are designed to be as short as possible and are insulated, as shown at55 in FIG. 4, to maximize heat energy delivered to the turbines 16, 22.

It is possible for the power turbine 16 to be rotatably supportedseparate from the turbocharger 18 and close coupled to the engine 12 asshown in FIGS. 1 and 4. Alternatively, the power turbine 16 may rotateon a tubular shaft 138 which is coaxial with the turbocharger core shaft24 as shown in FIG. 2. In this arrangement, however, it is preferable tohave the power turbine 16 and its tubular shaft 138 rotate independentlyof the shaft 24 and its associated compressor 20 and turbine 22. In thisway, the turbocharger compressor 20 and the turbine 22 can turbochargethe engine 12 independently of the power turbine 16, which providesadditional drive to the fan 14 through the second drive transmission 36,the drive shaft 30 and the first drive transmission 32.

FIG. 3 illustrates an alternative approach to turbocompounding where theturbocharger turbine 22 is geared directly back into the enginedriveshaft 30. Such a system transmits the difference in power producedby the turbine 22 and that demanded by the compressor 20.

Under certain engine operating conditions, it is desirable to bypasseither the power turbine 16 or the turbine 22, or both. Waste gates 58and 60 serve these respective purposes. By selective operation of thewaste gates 58 and 60, the level of engine turbocharging (manifoldpressure boost) is controlled and a proper balancing of drive input fromthe combustion engine 12 and the power turbine 16 can be achieved

The air exhausted from the fan 14 which does not enter the compressor 20is directed through the duct 40 to the exhaust nozzle 56 as indicated byarrows 57. The net propulsive thrust of the engine system 10 is the sumof the thrust provided by the fan 14 which bypasses the compressor 20and the thrust provided by the exhaust exiting turbine 22.

A conventional heat exchanger 62 may be employed to cool the combustionengine 12. The first drive transmission 32 and the second drivetransmission 36 preferably comprise compact, lightweight speed reductionsystems which produce minimal loss of output power. In addition, thesecond drive transmission 36 may include an overrunning clutch (notshown) to prevent the engine 12 from driving the power turbine 16 and afluid coupling (not shown) to attenuate the effect of engine torsionalvibration upon the power turbine 16.

Having described the various structural features of the presentinvention, its advantageous operation will now be described. Aspreviously mentioned, high altitude, high speed flight has generallyutilized a turbo fan or turbo jet type propulsion system and thesesystems have been fuel inefficient compared with internal combustionengines.

DESCRIPTION OF PREFERRED EMBODIMENT

Characteristic of aircraft gas turbine powerplants, the power availableat altitude is significantly reduced from the power available at sealevel due to the effect of the low density air at altitude. As a result,for a given thrust requirement at high altitude, a gas turbine is wellover designed for operation at sea level and low altitude.

In comparison, a turbocharged internal combustion engine can maintainrated power from sea level to high altitude. Furthermore, due to thelower air consumption requirements of the internal combustion engine,the associated turbomachinery size and weight are substantially reducedas compared to a conventional gas turbine at the same thrust level.

It therefore becomes evident that a turbocharged internal combustionengine begins to achieve a definite weight and size advantage inaddition to more fuel efficient operation as compared to theconventional gas turbine at high altitude.

Furthermore, it is seen that the size of the associated turbomachineryfor the turbocharged engine approaches a point at high altitude where itbecomes advantageous to design the inlet compressor stage as a fan toact as the main propulsor in addition to providing a stage ofcompression for the engine turbocharger system. Otherwise, to supply airof sufficient density to the engine, rather large and heavyturbochargers would typically be required, adding to the bulk and weightof the aircraft engine.

The improvement of the present invention enables turbochargers ofsmaller weight and dimension to be employed in aircraft engine systemsfor high subsonic flight at high altitude. By arranging the turbochargercompressor downstream from the fan, and by enclosing the entire systemin a common duct, the compressor receives at its inlet air which isalready somewhat pressurized by the fan. Because of this, the work whichmust be done by the compressor is reduced, thereby enabling a smallercompressor than would otherwise be possible. Since the net increase inpower output realized by turbocharging is dependent upon the powerrequired to drive the compressor, and less power is needed to drive thecompressor of the present invention.

Additionally, the engine system of the present invention enjoys the fuelefficiency of an internal combustion engine, as well as the propulsiveefficiency of a ducted fan, which makes it conducive to high altitudehigh speed flight. Moreover, compared to a turbine engine providingequivalent thrust, the turbocharger air flow can be smaller due to thelower fuel to air ratio associated with an internal combustion engine,therefore providing a higher thrust specific air consumption (TSAC) withreduced size and weight. Also, more of the propulsive thrust generatedby the fan bypasses the turbocharger and proceeds directly to theexhaust nozzle.

The combination of fuel efficient internal combustion engine with lowheat rejection features, turbocompounding, and a ducted fan propulsorwith a co-axial turbocharger provides a flight propulsion system forhigh subsonic flight at altitudes through the upper limits of thetropopause into the lower levels of the stratosphere with improvedefficiencies and lighter weight as compared to conventional gas turbinepowerplants.

The foregoing detailed description of the preferred embodiment has beengiven for clearness of understanding only, and no unnecessarylimitations should be understood therefrom. Some modifications will beobvious to those skilled in the art to which the invention pertains,without deviation from the spirit of the invention as defined by thescope of the appended claims.

We claim:
 1. A propulsion engine system for aircraft comprising:aninternal combustion engine including a rotatable drive shaft, an airinlet and an exhaust outlet; a fan for generating thrust, said fan beingrotatably mounted in a duct and drivingly connected to said drive shaft;a turbocharger including a turbine having an inlet and an outlet and acompressor having an inlet and an outlet, wherein said turbine isarranged for rotation with said compressor, and said turbine inlet isconnected to said exhaust outlet and said compressor outlet is connectedto said air inlet of said combustion engine; means for selectively,drivingly connecting said turbine to said drive shaft including aturbocharger shaft common to said turbine and said compressor, andreduction gearing between said turbocharger shaft and said drive shaft;wherein said compressor inlet is disposed in said duct downstream fromsaid fan to receive at least a portion of the air thrust from said fan.2. The invention as defined in claim 1 wherein said internal combustionengine is disposed adjacent said turbocharger.
 3. The invention asdefined in claim 1 wherein said exhaust outlet and said turbine inletand intermediate connections are insulated to minimize energy loss. 4.The invention as defined in claim 1 wherein said fan is multistage. 5.The invention as defined in claim 1 wherein said compressor ismultistage.
 6. The invention as defined in claim 1 wherein said turbineis multistage.
 7. The invention as defined in claim 1 and comprising anintercooler disposed between said compressor and said internalcombustion engine.
 8. The invention as defined in claim 1 and comprisinga heat exchanger for cooling said internal combustion engine.
 9. Theinvention as defined in claim 1 wherein said fan and said compressor arecoaxial.
 10. The invention as defined in claim 1 wherein said turbine isgeared directly back into said engine drive shaft.